- Executive Meddling:
- Hekikai was originally thought of before Karin. Kagesaki-sensei planned it while she was in middle school but had to change her original idea of Hekikai because the executive didn't like it.
- The manga was cut short and ended at 13 volumes with a rushed finale.
- Iron Woobie: Seine after killing her first mermaid; she stopped the crying for Simon's death and became an Action Girl Broken Bird and since then who knows how many things she has took with a stoic face.
- Hollywood Pudgy: Nagisa apparently consideres herself fat and she has taken summer diets that ultimately fail, but she isn't more fat than the rest of the cast.
- Ho Yay: Tsukasa was meant to be gay for Tatsuya or at least was pointed to be, as seen in this volume 2's omake. Though, now he's a wholesome fraternal figure to him, some can't stop see the Ho Yay between them. Even Minagi point it once!
- Moe: Sheila. Woobie? Check. Cute Mute? Check. A little bit of lost and you can't avoid want to protect her? Damn check.
- They Wasted a Perfectly Good Plot: Shizuki's war against Demiurge, her identity as the goddess Pistis Sophia and the past of the twins, and also the thing about if the death of Tatsuya's parents was really an accident or a murder.
- Ugly Cute: Croquet, Tatsuya's dog.
- Viewer Gender Confusion: At first look, it's difficult to know that Bella is really a female mermaid.
- The Woobie:
- Sheila. Her sisters are complete Jerkass towards her.
- Young Seine counts too. Her childhood before Simon was crap.
YMMV / Hekikai no AiON